Sensor part for installation in medium-voltage cable compartments and a device for measuring a voltage in medium-voltage circuits comprising such sensor part

ABSTRACT

Sensor part for installation in medium-voltage cable compartments, which sensor part comprises a voltage divider based on the capacitive divider principle, which voltage divider comprises: —a first capacitor, comprising an elongate primary conductor wrapped in a dielectric material and an elongate conducting shield arranged around the dielectric material, which first capacitor has a first capacitance rating; —a second capacitor, having a second capacitance rating, which second capacitor further comprises a first lead conductively connected with the conducting shield of the first capacitor and a second lead conductively connected to a common reference, such as earth; —a voltage output line, conductively connected with the conducting shield of the first capacitor; wherein the second capacitance rating is larger than the first capacitance rating, so that when during use the primary conductor is conductively connected with a live circuit carrying an alternating current, a measurement of a voltage between the common reference and the voltage output line can be taken as a ratio of the voltage between the live circuit and the common reference.

The invention relates to a sensor part for installation inmedium-voltage cable compartments, which sensor part comprises a voltagedivider based on the capacitive divider principle, which voltage dividercomprises:

-   -   a first capacitor, comprising an elongate primary conductor        wrapped in a dielectric material and an elongate conducting        shield arranged around the dielectric material, which first        capacitor has a first capacitance rating;    -   a second capacitor, having a second capacitance rating, which        second capacitor further comprises a first lead conductively        connected with the conducting shield of the first capacitor and        a second lead conductively connected to a common reference, such        as earth;    -   a voltage output line, conductively connected with the        conducting shield of the first capacitor;

wherein the second capacitance rating is larger than the firstcapacitance rating, so that when during use the primary conductor isconductively connected with a live circuit carrying an alternatingcurrent, a measurement of a voltage between the common reference and thevoltage output line can be taken as a ratio of the voltage between thelive circuit and the common reference.

Medium voltage circuits in circuit breaker applications typically needto be instrumented for monitoring, switching and optimization. Due tothe dangerously high voltages involved, human interventions on livecircuits are strictly prohibited. In order to bring the medium voltagelevels within safe measurement ranges, instrument transformers such ascurrent transformers and voltage transformers are commonly applied. Thebulky size of these transformers make installation hard and costly.

Present voltage and current transformers can, due to the usage of ironcores, go into saturation mode for their higher side operating voltageand current. Voltage transformers mostly go into saturation due to highferro magnetic currents. When a current transformer saturates, this canlead to failure in protection and power measurement devices.

Furthermore, varying temperature of current and voltage transformersyields non-linear output. High load currents are a factor in producingnon-linear output for measurements as well.

Even when, instead of voltage transformers, voltage dividers using thecapacitive divider principle are applied, the temperature variations inthe divider cause the output to be non-linear, making measurementsinaccurate.

It is an object of the invention to reduce or even remove theabove-mentioned disadvantages.

This object is achieved according to the invention with a sensor partaccording to the preamble, which is characterized in that the sensorpart further comprises:

-   -   a temperature sensor with a temperature output line, arranged in        heat conducting connection with the first capacitor for        measuring the temperature of the first capacitor.

By incorporating a temperature sensor, such as for example a platinumresistance thermometer (PRT), in heat conducting connection with thefirst capacitor, the temperature of the capacitor can be accuratelyestablished.

The capacitance value of the capacitor is temperature dependent, so byaccurately determining the temperature of the capacitor, the actualvoltage division can be calculated.

Through the heat conducting connection between the temperature sensorand the capacitor, an accurate reading of the temperature sensor can bedetermined.

The long term stability of a PRT makes it highly suitable forapplication in medium voltage cabinets, since it ensures a long operableservice life of the sensor part.

The area of the primary conductor enclosed by the shield forms a firstcapacitance plate of the first capacitor, the conducting shield formsthe second capacitance plate of the first capacitor and the dielectricmaterial separating both conductors completes the capacitor.

In another embodiment of a sensor part according to the invention, thedielectric material further extends radially from the conducting shieldforming an insulating housing or bushing.

The dielectric material part of the first capacitor can further extendto form a bushing. The first capacitor thus being practically immersedinto the insulating material forming the bushing.

Another embodiment of a sensor part according to the invention, is anembodiment in which the temperature sensor is arranged within thedielectric material.

The temperature sensor can be included within the insulating material,so that accurate temperature readings can be obtained of the firstcapacitor directly, or indirectly through the bushing. The bushing is inheat conducting connection with the capacitor, not in the least sincethe bushing forms an integral part of the capacitance.

In another embodiment of a sensor part according to invention, the ratioof the first capacitance rating and the second capacitance rating isapproximately 1:300, 1:220 or 1:85, for bringing respectively 35 kV, 26kV or 10 kV within a safe measuring range of 0-120V.

One of the goals for applying instrument transformers is to allow safemeasurement on medium voltage circuits. For this, the transformation ordivision ratio needs to be chosen according to the voltages in which thesensor part will be deployed. Typical secondary proportional voltagesare under 120V. It is known for the person skilled in the art how todetermine an appropriate ratio in line with for example IEEE or otherregulations.

Another, current sensing, embodiment of a sensor part according to theinvention, is an embodiment wherein the sensor part further comprises:

-   -   a current output line;    -   a printed circuit board (PCB) Rogowski current sensor comprising        a Rogowski coil, wherein the Rogowski coil is conductively        connected with the current output line.

By extending the sensor part with a current sensor, the sensor partfinds even further application in circuit breaking applications. Byreplacing the traditionally deployed current transformer with currentsensing employing a Rogowski coil, the sensor can be constructedrelatively compact. The Rogowski coil furthermore non-saturating andprovides good linearity. The Rogowski coil is formed by conductivetraces on a circuit board. When the sensor part is in use, the Rogowskicoil is placed around a primary conductor carrying an alternatingcurrent. The coil voltage between the first and the second end of theRogowski coil provides a current signature of the current flowing in theprimary conductor. The coil voltage is typically in the millivoltsrange, so the signal is typically further processed at a later stage.The coil inductance displaces the current signature in the coil voltage90 degrees with respect to the current in the primary conductor. Laterprocessing can further include phase-shifting to correct for the lag inthe coil voltage signal.

In another current sensing embodiment of a sensor part according to theinvention, the temperature sensor is arranged in heat conductingconnection with the Rogowski coil, for measuring the temperature of theRogowski coil.

By ensuring heat conducting connection between the Rogowski coil, thetemperature sensor and the first capacitor, a measurement on thetemperature output line can be used to compensate for temperaturevariations in the measurements from the voltage output line and thecurrent output line.

Alternatively, the sensor part can comprise a second temperature sensorwith a second temperature output line, arranged in heat conductingconnection with the Rogowski coil for measuring the temperature of theRogowski coil. This can be useful when a heat conducting connectionbetween the Rogowski coil and the first capacitor cannot easily beachieved. A single temperature measurement can then not be used tocorrect the measurements. This can be resolved by adding anothertemperature sensor specific for the Rogowski coil.

The invention further relates to a device for measuring a voltage inmedium-voltage circuits, comprising a sensor part according to theinvention, wherein the device further comprises a converter part, whichconverter part comprises a controller for applying a temperaturecorrection to a voltage measurement taken between the voltage outputline and the common reference, which controller comprises a correctedvoltage signal determined by taking the voltage measurement and applyinga correction based on a temperature measurement taken from thetemperature output line, for compensating for temperature relatedvariation of the first capacitance rating.

An embodiment of such device according to the invention applies postprocessing on the output lines, so that the measurements can be useddirectly in further processing. The converter part, which can forexample be connected with the output lines of the sensor part byshielded cables, can comprise a variety of sub-components such as:

-   -   analog input filter(s)    -   analog to digital converter(s) (ADC)    -   temperature and/or electronic biasing circuit(s)    -   microcontroller(s)    -   digital to analog converter(s) (DAC)    -   communication interface(s)    -   volatile or persistent storage(s)

The controller part applies corrections based on the temperaturecoefficient of the capacitors of the voltage divider of the sensor part,to get more accuracy on the corrected voltage signal. The controller canapply further calibration data as well, which calibration can forexample be stored in persistent storage in the converter part. Thecontroller can comprise one or more of the aforementionedsub-components.

The other sub-components can be applied to provide clean, noise freeanalog signals to the controller. Analog filtering can for exampleconsist of a low-pass filter for filtering out unwanted noise orfrequencies in the signal, a buffer (op-amp circuit) for matchingcircuit impedances, anti-aliasing filter (RC filter) for matching thesignal with the characteristics of an attached ADC. It will be apparentfor the person skilled in the art how to select and arrange thesub-components.

Another embodiment of a device according to the invention comprises acurrent sensing sensor part according to the invention, wherein thecontroller further comprises a corrected current signal determined bytaking a secondary voltage measurement between the current output lineand the common reference and applying a correction based on atemperature measurement taken from the temperature output line, forcompensating for temperature related variation of the Rogowski coilvoltage.

Measurements taken from the Rogowski coil can be corrected fortemperature induced variations in the levels of the current output linebased on a measurement from the temperature output line, or, if soequipped, from the second temperature output line. Calibration data canalso be used to further compose the corrected current signal.

In another embodiment of a device according to the invention, thecontroller comprises updatable firmware for storing and adaptingtemperature related corrections.

Temperature related coefficients or other calibration data can change.Also variations in capacitance, temperature coefficients, coilsensitivity, etc., introduced during manufacture, make it worthwhile tobe able to program the converter part with the appropriate values. Thiscan for example be done during production, or at a later stage, forexample when the sensor part is calibrated.

In a further embodiment of the device according to the invention theconverter part comprises a cascade of comparators to apply a correctionfactor depending on in which temperature range the temperaturemeasurement is. The correction factor is applied to the voltage andcurrent.

In yet another embodiment of the device according to the invention afunction for the correction factor is implemented in the converter part,which function is dependent on the temperature and is a 1, 2 or higherorder function between temperature and correction factor.

Also an embodiment of a device according to the invention, is a devicewherein the converter part further comprises a communication port forreceiving firmware and/or communicating the corrected voltage signal orthe corrected current signal.

In order to support field-updatability of the controller and/or tocommunicate with the converter part for obtaining corrected signals orother derivatives, such as frequency, active/reactive power, powerfactor, etc., a communication port, such as a serial port, SPI lines,USB, Ethernet or other communication ports can be arranged in theconverter part.

These and other features of the invention will be elucidated inconjunction with the accompanying drawings.

FIG. 1 shows a side-by-side view of an embodiment of a sensor partaccording to the invention with marked out inner components and aschematic representation of the same sensor part.

FIG. 2 shows a cross-sectional view of the embodiment of the sensor partaccording to the invention.

FIG. 3 shows a schematic view of a device according to the invention.

FIGS. 4A and 4B show an embodiment of a current sensor for use in thedevice according to the invention.

FIGS. 5A, 5B and 5C show a diagram of a method performed in theconverter part of the schematic view of FIG. 3 .

FIG. 1 shows a side-by-side view of an embodiment of a sensor part 1according to the invention with marked out inner components and aschematic representation of the same sensor part. The sensor part 1 inthe form of a bushing, has the primary conductor 2 running through thecore of the bushing 1. The first capacitor 3 is formed by the elongateprimary conductor 2, the dielectric material 4 which surrounds theprimary conductor 2 and the conductive shield 5 enclosing the primaryconductor 2. The conductive shield 5 forms one plate of the capacitance.The part of the primary conductor 2 enclosed by the conductive shield 5forms the other plate.

Conductively connected with the conducting shield 5 is a voltage outputline 6, as well as the first lead 7 of the second capacitor 8. Thesecond lead 9 of the second capacitor 8 is for conductively connecting,during use of the sensor part, to a common reference 10, such as earth.

Close to the conducting shield 5 and the primary conductor 2, atemperature sensor 11 is arranged. It is in heat conducting connectionwith the first capacitor 3. The temperature output line 12 allowsmeasurements of the temperature sensor to be taken. While in this figurethe second capacitor 8 is placed outside the bushing 1, which istypically arranged in an auxiliary component, such as a converter part.

FIG. 2 shows a cross-sectional view of the embodiment of the sensor part1 according to the invention. The dielectric material 4 wrapped aroundthe primary conductor 2 en further extending radially to form thehousing of the bushing 1. The conducting shield 5 formed around theprimary conductor 2 is conductively connected with the voltage outputline 6 as well as with the first lead 7 of the second capacitor 8.

The figure clearly shows the dielectric material 4 continuing betweenthe conducting shield 5 and the primary conductor 2. The region ismarked where the primary conductor 2 is enclosed in the conductingshield 5, forming the first capacitor 3.

The temperature sensor 11 arranged near the capacitor 3 is connectedwith the temperature output line 12.

FIG. 3 shows a schematic view of a device 20 according to the invention.The sensor part 21 has a voltage sensor 22 according to the capacitivedivider principle, a temperature sensor 23 and a current sensor 24. Ashielded interface cable 25 connects the sensor part 21 with theconverter part 26. The converter part 26 comprises an analog filteringstage 27 which is connected with a controller 28. A communication port29 allows the converter part 26 to be connected with further devices.The interface cable 25 has several cores, such as two cores for thetemperature sensor 23, two cores for the current sensor 24 and a singlecore for the voltage sensor 22.

The corrected voltage signal 30 and the corrected current signal 31 aresupplied by the controller 28.

FIGS. 4A and 4B show an embodiment of a current sensor 24 for use in thedevice according to the invention. The current sensor 24 is a Rogowskicoil, known from EP 3502714, which has a printed circuit board 42 with acentral passage opening 43 through which a conductor C extends.

The Rogowski coil 24 has a first winding composed out of tracks 44, 45and vias 46 and a second, return winding composed out of tracks 47, 48and vias 49. The first winding 44, 45, 46 and the second, return winding47, 48, 49 are arranged in series at the coupling 50, where the firstwinding is electrically connected to the second winding.

The conductor C generates an magnetic field F, which causes a current Iin the first winding 44, 45, 46 and the second, return winding 47, 48,49. (see FIG. 4B). As the first and second winding have oppositedirection of rotation around the respective axis and because the secondwinding 47, 48, 49 returns back towards the start 51 of the firstwinding 44, 45, 46, the current I in the first and second windingssupport each other, such that the sensitivity of the Rogowski coil 24 isincreased.

FIGS. 5A-5C shows a diagram of a method performed in the converter part26. The method starts with powering on or starting the method at 100.This ensures that the analog to digital converter is initialized at 101,the microcontroller is initialized at 102 and the digital to analogconverter for the external interface is initialized at 103.

Then the output of the voltage sensor 22 and the output of the currentsensor 24 are read at 104. These readings together with the output 105of the temperature sensor are converted into a digital signal at 106.These digital signals are fed to a calibration module 107, which isexplained in more detail in FIG. 5B.

The output of the calibration module 107 is then fed to a compensationmodule 108, which compensates for temperature. The output of thecompensation module 108 is the provided to an external interface 109 anda digital interface 110, such that the method ends at 111.

FIG. 5B shows the method steps performed in the calibration module 107in more detail.

At 120 the configuration of the calibration module 107 is started. Thenthe configuration factor for the voltage signal is determined at 121,which is then applied to the voltage signal at 122 to obtain the linevoltage of the primary conductor 2. For the current, the configurationfactor is determined at 123 and then applied at 124 to obtain the linecurrent. And the temperature configuration factor is determined at 125and then applied on the temperature signal to obtain the bushingtemperature at 126.

FIG. 5C shows the method steps performed in the compensation module 108.Although a lookup table would be obvious to use, the electronics toimplement such a lookup table in a high voltage sensor according to theinvention, would be costly. Therefor, an alternative temperaturecompensation is applied, which allows the use of low cost electronics.

First of all the bushing temperature provided by the configurationmodule 107 is compared to a reference temperature, for example 25° C.,at comparator 130. If the bushing temperature is exactly this referencetemperature, then the temperature compensation is skipped and the methodsteps end at 131.

At 132 it is checked if the bushing temperature is lower than thereference temperature. If so, the method continues to the nextcomparator 133, and if not, the method continues at the comparator 134.

At the comparator 133 it is checked whether the bushing temperature ishigher than 5° C., so whether the temperature is thus between 5° C. and25° C. If so, a first correction factor L1 is provided at 135 andapplied at 131, where the method steps end.

Similarly, the comparator 134 checks whether the bushing temperature isbetween 25° C. and 45° C., and if so, a correction factor H1 is providedat 136.

The comparator 137 compares the temperature with the range 4° C. and−15° C. and provides the factor L2 at 138 if the bushing temperature iswithin said range.

The comparator 139 compares the temperature with the range of −15° C.and −40° C. and applies the factor H3 at 140 if there is a match.However, if there is no match with this last temperature range, then themethod continues to step 141 to wait for a new reading of the bushingtemperature, as apparently the current bushing temperature was a fallsreading.

For the temperatures above the reference temperature of 25° C., thecomparator 142 checks for the range 45° C. and 65° C. and applies afactor H2 at 143, and the comparator 144 checks for the range 65° C. and90° C. and provide the factor H3 if there is a match.

It is clear for a person skilled in the art that the number ofcomparators in this cascade of comparators can be altered to provide ahigher or lower compensation accuracy. Also the temperature rangesassigned to the comparators can be defined based on the requirements ofthe specific case and sensor.

1.-9. (canceled)
 10. A device for measuring a voltage in medium-voltagecircuits, the device comprising: sensor part for installation inmedium-voltage cable compartments, which sensor part comprises a voltagedivider based on the capacitive divider principle, which voltage dividercomprises: a first capacitor, comprising an elongate primary conductorwrapped in a dielectric material and an elongate conducting shieldarranged around the dielectric material, which first capacitor has afirst capacitance rating, wherein a part of the primary conductorenclosed by the conducting shield forms a first capacitance plate of thefirst capacitor and the conducting shield forms a second capacitanceplate of the first conductor; a second capacitor, having a secondcapacitance rating, which second capacitor further comprises a firstlead conductively connected with the conducting shield of the firstcapacitor and a second lead conductively connected to a commonreference, such as earth; and a voltage output line, conductivelyconnected with the conducting shield of the first capacitor; wherein thesecond capacitance rating is larger than the first capacitance rating,so that when during use the primary conductor is conductively connectedwith a live circuit carrying an alternating current, a measurement of avoltage between the common reference and the voltage output line can betaken as a ratio of the voltage between the live circuit and the commonreference, wherein the sensor part further comprises a temperaturesensor with a temperature output line, arranged in heat conductingconnection with the first capacitor for measuring the temperature of thefirst capacitor, and wherein the device further comprises a converterpart, which converter part comprises a controller for applying atemperature correction to a voltage measurement taken between thevoltage output line and the common reference, which controller comprisesa corrected voltage signal determined by taking the voltage measurementand applying a correction based on a temperature measurement taken fromthe temperature output line, for compensating for temperature relatedvariation of the first capacitance rating, wherein the converter partcomprises a cascade of comparators to apply a correction factordepending on in which temperature range the temperature measurement is.11. The device according to claim 10, wherein the dielectric materialfurther extends radially from the conducting shield forming aninsulating housing or bushing.
 12. The device according to claim 10,wherein the temperature sensor is arranged within the dielectricmaterial.
 13. The device according to claim 10, wherein the ratio of thefirst capacitance rating and the second capacitance rating isapproximately 1:300, 1:220 or 1:85, for bringing respectively 35 kV, 26kV or 10 kV within a safe measuring range of 0-120V.
 14. The deviceaccording to claim 10, wherein the sensor part further comprises: acurrent output line; a printed circuit board Rogowski current sensorcomprising a Rogowski coil, wherein the Rogowski coil is conductivelyconnected with the current output line.
 15. The device according toclaim 14, wherein the temperature sensor is arranged in heat conductingconnection with the Rogowski coil, for measuring the temperature of theRogowski coil.
 16. The device according to claim 14, wherein thecontroller further comprises a corrected current signal determined bytaking a secondary current measurement between the current output lineand the common reference and applying a correction based on atemperature measurement taken from the temperature output line, forcompensating for temperature related variation of the Rogowski coilvoltage.
 17. The device according to claim 10, wherein the controllercomprises updatable firmware for storing and adapting temperaturerelated corrections.
 18. The device according to claim 10, wherein theconverter part further comprises a communication port for receivingfirmware and/or communicating the corrected voltage signal or thecorrected current signal.